We propose structural experiments addressing two different problems: (1) the pathogenic mechanism of exotoxin A of Pseudomonas aeruginosa, and (2) the structure of the T4 single-stranded DNA binding protein, alias gene 32 protein. Exotoxin A is a major determinant of lethality of P. aeruginosa; as such, it has been intensively studied (i) with regard to infection by and immunization against P. aeruginosa; (ii) as a system for understanding the actions of bacterial toxins, and (iii) as a potential effector moiety for constructing targeted cytotoxic agents. We are solving the x-ray crystallographic structure of exotoxin A; we have recently traced the polypeptide backbone of the molecule. The structure--the first determined for a major bacterial toxin--suggests a strategy for solving the membrane translocation mechanism of the toxin, employing and integrating (i) further crystallographic experiments; (ii) electron microscopy of toxin-membrane complexes; (iii) photoaffinity labelling of the toxin within membranes; and (iv) theoretical modelling of interaction of the toxin with hydrophobic environments. The T4 gene 32 protein is intimately involved in DNA replication, recombination, and repair. It also has an unusual mechanism of autoregulation: it is a translational repressor of its own synthesis. We have developed a strategy for solving the structure of the gene 32 protein. We plan to proteolytically modify the protein to obtain crystals suitable for x-ray diffraction, and to co-crystallize the protein with DNA oligomers. The three-dimensional structure of the protein should provide a foundation for interpreting current biochemical and genetic data on its function; potentially this system will provide a prototype model for involvement of DNA binding proteins in nucleic acid metabolism and translational regulation in higher organisms.